1. Technical Field
The present invention relates to a method and structure for reducing chip carrier flexing during thermal cycling.
2. Related Art
A semiconductor chip may be mechanically and conductively coupled to a chip carrier by having conductive contacts on the chip (e.g., Controlled Collapse Chip Connection solder balls) solderably coupled to the top surface of the chip carrier. During processing steps that elevate the temperature of the chip carrier, such as during a reflow step for solderably joining the chip to the chip carrier, a spatial distribution of coefficient of thermal expansion (CTE) due to material inhomogeneities within the chip carrier may cause the chip carrier to bow (or flex) upward or downward and thus deviate from planarity. For example, there may be large copper pads on the bottom of the chip carrier to accommodate a ball grid array of solder balls for coupling the chip carrier to a circuit card, and smaller amounts of copper, such as in the form of copper circuitization and/or copper pads for joining a chip to the chip carrier on the top surface of the chip carrier. The spatial distribution of CTE, and consequent bowing or sagging of the chip carrier when the chip carrier is heated, is a result of copper imbalance between the top and bottom surfaces of the chip carrier combined with volumetric distribution within the chip carrier of materials having different magnitudes of CTE.
The preceding chip carrier flexing problem increases in severity if the chip carrier is made of compliant material, such as compliant organic material which cannot be easily handled (e.g., a material having an elastic modulus of less than about 300,000 psi). An organic chip carrier that is highly compliant may benefit from a rigid xe2x80x9cstiffener ringxe2x80x9d bonded to an outer perimeter of the top surface of the chip carrier in order to enhance the structural characteristics of the chip carrier. That is, the stiffener ring makes the chip carrier more mechanically stable and thus easier to handle. Unfortunately, the stiffener ring acts as a mechanical clamp on the outer perimeter of the chip carrier that constrains outer portions of the chip carrier from expanding, particularly when subjected to elevated temperature. In contrast, center portions of chip carrier at which chips are typically attached, are not constrained by the stiffener ring. Thus, expansion of the central portions, when heated, accentuates the chip carrier bowing by causing a distinct upward bulge in the central portion of the chip carrier top surface.
An adverse consequence of chip carrier bowing, particularly when a stiffener ring is used with a compliant organic chip carrier, is unreliable coupling of a chip to the chip carrier, as illustrated in FIGS. 1 and 2. FIG. 1 shows a semiconductor chip 10 resting on an organic chip carrier 20 at ambient room temperature, wherein a top surface 14 of the chip carrier 20 is flat, and wherein solder balls 11, 12, and 13 on the semiconductor chip 10 are in conductive contact with solder bumps 24, 25, and 26 at the conductive pads 17, 18, and 19 on the top surface 14 of the chip carrier 20, respectively. A stiffener ring 15 is bonded to the outer perimeter of the chip carrier 20 by an interfacing adhesive 16. FIG. 2 shows the chip carrier 20 of FIG. 1 under temperature elevation, such as when solder from the solder bumps 24, 25, and 26 is reflowed around the solder balls 11, 12, and 13 in an attempt to conductively join the solder balls 11, 12, and 13 to the conductive pads 17, 18, and 19, respectively. At the elevated temperature, the center the chip carrier 20 is bows (or bulges) upward in the direction 22, such that the solder balls 11 and 13 are no longer in conductive contact with the conductive pads 17 and 19, respectively. Thus, the chip carrier flexing impairs the ability to reliably join a chip to a chip carrier. The bowing B may exceed 2 to 3 mils during solder reflow.
A method and structure is needed for reducing or eliminating flexing of a compliant organic chip carrier in an elevated temperature environment, and particularly when solder is reflowed around solder balls of a semiconductor chip for joining the semiconductor chip to the chip carrier.
The related art does not teach or suggest how to reduce flexing of a stiff chip carrier during thermal cycling. As a result, a stiff chip carrier may experience flexing that leads to cracking during thermal cycling.
A method and structure is therefore needed for reducing chip carrier flexing during thermal cycling.
The present invention provides an electronic structure, comprising:
a chip carrier having an elastic modulus of at least about 3xc3x97105 psi, wherein there is no stiffener ring coupled to a peripheral portion of the chip carrier; and
a semiconductor chip coupled to a central portion of the chip carrier.
The present invention provides a method of forming an electronic structure, comprising:
providing a chip carrier having an elastic modulus of at least about 3xc3x97105 psi, wherein there is no stiffener ring coupled to a peripheral portion of the chip carrier; and
coupling a semiconductor chip to a central portion of the chip carrier.
The present invention improves an ability of a stiff chip carrier to accommodate stresses thermally induced in the chip carrier during thermal in order to reduce or eliminate flexing of the chip carrier during the thermal cycling.